1. Field of the Invention
The present invention relates to a tire information detecting apparatus for monitoring the pneumatic pressure of tires
2. Description of the Related Art
A tire information detecting apparatus according to the related art will be described with reference to FIG. 3. A controller includes at least one generator G1 for generating radio frequency with respect to a carrier signal f1 at a microwave frequency band of about 2.4 GHz. The carrier signal f1 is modulated by at least one low frequency signal f2 which is generated by a generator G2 and preferably at a frequency band in the range of 1 to 30 MHz. As a result of the modulation, a desired supply frequency is generated. A signal of the generated frequency is amplified and transmitted via an antenna A1 disposed around a tire.
Preferably, the modulation means an amplitude modulation. According to the above-mentioned modulation method, a sideband is generated along the carrier frequency, at both right and left sides in a spectrum, for example, at portions of f1+f2 and f1−f2 by the amplitude modulation. When a plurality of frequencies f2 is used, by summing them, the sideband shown in the drawing generates a spectrum. The modulation is switched off by an electronic switch S1, and the electronic switch S1 is periodically controlled by a timer T1.
The tire includes at least one measured value transmitter MG1 (transponder), the measured value transmitter MG1 includes at least one antenna A2, a receiver having at least one diode, and a crystal resonator Q1 which is excited by a received modulation signal. The crystal resonator Q1 changes the resonant frequency on the basis of the tire pneumatic pressure, and again the crystal resonator Q1 is coupled to its own modulator diode or a mixer diode D2, or preferably, a varactor diode which allows parametric gains to be used. Further, the frequency varies on the basis of the measured values. The modulation is switched off at time t1 by the switch S1. Immediately after that, a receiver E1 becomes active at time t2 which is 1 μs after the time t1.
When the modulation of the supply frequency is switched off, the crystal resonator Q1 further excites for about 1 ms. Since the carrier is still in presence, the supply frequency is modulated by the modulation diode D2. However, this happens when only a modulation frequency f2 excites the crystal resonator G1, that is, when the modulation frequency f2 substantially corresponds to a predetermined measured value. Since a supply signal is not modulated by the antenna A1 which can cause interference, the receiver recognizes a modulated signal of an antenna A3 by using an antenna A4, thus a measured value can be obtained through the modulation. When the modulation is not performed or it is performed a little, a predetermined measured value can be repeatedly sampled (see Japanese Patent No. 3494440).
In the transponder MG1, an amplitude modulation wave is detected by a diode D1 so that a modulation wave of the modulation frequency f2 is detected, and then the crystal resonator Q1 is excited by the detected modulation wave. However, it is necessary to increase the modulation degree of the amplitude modulation wave, in order to cause the crystal resonator Q1 to sufficiently excite. On the other hand, when the modulation degree increases to cause the crystal resonator to easily excite, the modulation wave is greatly distorted, thereby causing a spurious problem. Further, if the modulation degree decreases in order to escape the problem, the modulation level is lowered, whereby it is impossible to cause the crystal resonator to sufficiently excite.